With the development of mobile communication technology and the evolution of communication networks, a variety of mobile communication systems have been developed. In order to solve the problem of global roaming between the mobile communication systems, the International Mobile Telecommunication (IMT)-2000 systems have been developed. The IMT-2000 systems are classified into a Code Division Multiple Access (CDMA) 2000-based synchronous system and a Wideband Code Division Multiple Access (WCDMA)-based asynchronous system.
Furthermore, in order to support global roaming between the mobile communication systems, a mobile communication terminal capable of being used in both a synchronous type system and an asynchronous type system, that is, a Dual Band Dual Mode (DBDM) mobile communication terminal, has been developed. Using the mobile communication terminal, different types of services can be respectively used in an asynchronous system area and a synchronous system area.
Currently, the asynchronous mobile communication network is being constructed in regions having high demand for service and, therefore, the synchronous mobile communication network is evolving such that the service area thereof includes the service areas of an asynchronous system. In such a process, handover between the systems is necessary so as to provide continuous service in the case where a user moves between an asynchronous mobile communication system and a synchronous mobile communication system.
In this environment, roaming between a synchronous type system and an asynchronous type system becomes the most critical issue. Conventionally, a problem occurs in that a data interruption time of more than 10 seconds occurs and, therefore, it is difficult to provide normal data service, in consideration of modem initialization, initial call establishment, data call establishment, and MIP address registration time, when a mobile communication terminal receiving service in an asynchronous mobile communication network performs handover to a synchronous mobile communication network, such as a CDMA2000 1x/Ev-Do network, at the time of performing a data call.